1. Field of the Invention
The present invention relates to high-volume, fully dense, multi-component silicon nitride monoliths with improved properties, and to a method of making same.
2. Background Art
The unique properties of silicon nitride ceramics have made them attractive not only for use as cutting tools, engine components, ball bearings, wear parts, but also for pump parts and riser tubes. The extremely good thermal shock resistance against non-ferrous melts predestines dense silicon nitride also for use in non-ferrous metallurgy where in many cases high-volume, large-sized components of high reliability are required.
The production costs are a main problem in the application of silicon nitride ceramics. This is connected on the one hand with the high costs of raw material (submicron particle sized Si3N4 powder) and on the other hand with the component manufacturing technologies. Thus, if a large ceramic Si3N4 body is directly sintered or hot pressed in conventional fashion there will be a tendency for the body to display a large variation in sintered density and strength. In many instances it is impractical to fabricate large sized components in one piece and individual parts cannot be assembled and held together by mechanical means alone. There is, therefore, a considerable need for economical, non-mechanical methods to join Si3N4 ceramics.
Joining methods for Si3N4 ceramics can be grouped into the following categories: (i) solid state bonding with and without interlayers, (ii) direct joining of liquid phase sintered or hot pressed materials via hot pressing, (iii) bonding by liquid wetting and capillarity (pressureless, with for example silicate or oxynitride glasses or metal brazes), and (iv) joining by ceramic processing techniques. Nakamura et al., [“Joining of Silicon Nitride Ceramics by Hot Pressing”, J. Mat. Sci. 22 (1987), 1259-1264] investigated joining of dense, hot pressed Si3N4 ceramics containing alumina and yttria as densification aids. Uniaxial pressure was applied at high temperature during the joining process. A polyethylene sheet of thickness 40 μm was used as a joining agent between the polished specimen surfaces to be joined. The joint strength was measured by four-point bend tests using test bars cut from joined HPSN couples perpendicular to the joining interfaces. The joint strengths increased with increases in joining temperature, joining pressure and holding time. The highest joint strength obtained was 567 MPa, which was about half the value of the mean strength of the original body. The formation of porous interfacial zones at the joints was considered to cause the reduction of the joint strength.
Especially the use of glass and metal interlayers may not produce seamless bonds because of the potential mismatch of the thermal expansion coefficients between the interlayer and the silicon nitride. In most cases the bond is neither as refractory nor as resistant to oxidation and corrosion attack as the base materials joined, i.e. joining seams exist, the joints being the performance limiting “weak links” in the multicomponent monolith. Moreover the categories of joining methods (i)-(iii) have the certain disadvantages (a) high cost for the required dense silicon nitride parts, (b) the long time required to grind and polish the individual parts to be joined at their joining surfaces, (c) proper alignment of parts is difficult to achieve, and (d) individual parts frequently fracture or deform while being joined.
On the other hand, method (iv), joining by ceramic processing techniques, seems capable of solving problems of weak joint strength and may produce excellent joints if properly performed.
State of the art in joining via ceramic processing techniques uses powder hot pressing or hot pressing of previously molded bodies to simultaneously densify and join ceramic parts. The initial work has focused on attaching components of dissimilar silicon nitride materials. According to the method which is disclosed in U.S. Pat. No. 3,854,189 to Ezis et al., a duodensity Si3N4 turbine rotor can be fabricated by hot-press bonding a reaction bonded Si3N4 (RBSN) blade ring to a previously hot pressed Si3N4 rotor hub. This process was later improved by simultaneously densifying the hot pressed Si3N4 rotor hub and bonding it to the RBSN blade ring, i.e. a predetermined amount of Si3N4 powder with a MgO additive was placed in the hub cavity and hot pressed to theoretical density while simultaneously bonding to the RBSN blade ring [Goodyear and Ezis: “Joining of Turbine Engine Ceramics”, pp. 113-153 in Advances in Joining Technologies, edited by J. J. Burke et al., Brook Hill Publ., Chestnut Hill, Mass., 1976]. In a similar way Gugel and Kessel [“Post Hot Pressing of Reaction Bonded Silicon Nitride”, pp. 515-526 in Ceramics for High Performance Applications II, edited by J. J. Burke et al., Brook Hill Publ., Chestnut Hill, Mass., 1978] have successfully fabricated a duodensity Si3N4 turbine rotor by simultaneously densifying a preformed RBSN hub and bonding it to a RBSN blade ring. However, the properties of the hot pressed to reaction bonded Si3N4 joints are limited by the inferior mechanical properties of the reaction bonded Si3N4.
Some success has been reported by Bates et al., [“Joining of Non-Oxide Ceramics for High Temperature Applications”, Am. Ceram. Soc. Bull. 3 (1990), 350-6] on joining of Si3N4 with itself using hot isostatic pressing (HIP) as the ceramic processing technique. In this case the parts to be joined were green compacts obtained by cold isostatic pressing of sinterable Si3N4 powder containing 4% by weight yttria (Y2O3). Three joining conditions were evaluated: self-bonded without filler material, and two cases where a filler material consisting of Si3N4 containing 4% by weight Y2O3 was used. Joining and simultaneous densification was accomplished by glass encapsulation HIP to 100% of theoretical density (100% TD). Microfocus X-ray radiography did not detect porosity or glass pockets at the joint. However, despite the use of a high pressure HIP-densification process, a significant drop in the average strength of self-bonded and interlayer-bonded Si3N4 parts was experienced relative to control (unjoined) HIPed Si3N4 parts.
The possibility to join partially sintered and devitrified Si3N4 bodies by ceramic processing techniques into an integral unit having a complex shape, a density higher than 98% of the theoretical and high joint strength at 1200° C. (>50 kp/mm2) was demonstrated in U.S. Pat. No. 4,172,107 to Nakamura et al. The basis of the method is pseudo-isostatically hot pressing (at 1780° C. with a relatively high pressure in the range of 350-450 bar and use of a pressure transmitting powder bed) an assembly of partially sintered and devitrified Si3N4 parts whereby densification and joining is simultaneously affected. The partially sintered Si3N4 bodies to be used by the process have a density of preferably 70-75% TD and are made from compacts of submicron Si3N4 powder with an admixture of yttria and alumina by heating at 1700-1750° C. in an aluminium nitride (AlN) powder bed for 90-250 mins. As can be seen from column 4, lines 15-22 of U.S. Pat. No. 4,172,107, the AlN powder bed induces crystallization of the amorphous binder phase whereby improved high temperature strength of the final composite structure and consequently its use as a turbine rotor is affected.
It has been reported from Tsuge et al. [“High Strength Hot Pressed Si3N4 With Concurrent Y2O3 and Al2O3 Additions”, Am. Ceram. Soc. Bull., 57 (1978), 424-431], in relation to this high-temperature presintering step, that for Si3N4 compositions containing 5 Y2O3-2Al2O3 (wt %) the preheating of compacts embedded in AlN powder at 1750° C. brings about devitrification of the glassy grain boundary phase i.e. leads to crystalline Si3N4.Y2O3 as the dominant grain boundary phase in the hot pressed Si3N4 bodies.
However, owing to the high raw material and process costs the joining method according to U.S. Pat. No. 4,172,107 is economically and technologically disadvantageous and is as yet unsuitable for mass production of large hot pressed Si3N4 monoliths.
The present invention differs from the teachings of U.S. Pat. No. 4,172,107 relating to (1) a low-pressure hot pressing densification of an assembly of low cost RBSN bodies without use of a pseudo-isostatic pressure transmitting medium, and (2) a hot pressed Si3N4 monolith containing a predominantly amorphous oxynitride or silicate glass as main component of the binder (grain boundary) phase, and having a combination of unique mechanical properties both in the joint areas and in the bulk of the material.